Here's how to upgrade Magento 2 to a later version. For example, if you are currently running 2.1.6 and you need to upgrade to 2.1.7. Make sure you are in your Magento 2 public_html directory before you start.
1) Put your store in maintenance mode:
php bin/magento maintenance:enable
2) Update the composer.json with the latest upgrade version:
composer require magento/product-community-edition 2.1.7 --no-update
3) Update the files:
composer update
4) Clear the var cache files:
rm -fr var/{cache,page_cache,generation}/*
5) Update the Database:
php bin/magento setup:upgrade
6) Run Magento compile command:
php bin/magento setup:di:compile
If you get memory issues, run this with more memory:
php -d memory_limit=4G bin/magento setup:di:compile
7) Turn off Maintenance mode:
php bin/magento maintenance:disable
8) Revisit your site admin and check the version number in the footer.